Not sure what it does if i change the value in open pht. Thank you!
Nothing - has no effect on playback whatsoever !
Also check this thread; https://forums.plex.tv/discussion/279851/advancedsettings-cache#latest
As if have done research on this.
It is my impression that it controls how much of the (remote) video stream is cached in memory.
It take this from studying the numbers displayed in the information overlay one gets after hitting i on the keyboard while video is streamed. The size of the buffered data increases with time up to a limiting value. After reaching the data transfer is throttled. That value scales with the cache size value selected in the preferences dialog. It works out to be close to the percentage value multiplied with the total size of the video stream.
As you can see I have no insights in the code and form my opinion solely from the observations I made.
I do not know why this design decision has been made, and one does not choose a fixed time value. Hence, if you select 2%, it will cache about 2 1/2 minutes of a typical 2 h feature movie. How much that is in MBytes depends of the data rate.